Question

I have a form for creating a new invoice with many items.

class Invoice < ActiveRecord::Base
  attr_accessible :project_id, :number, :date, :recipient, :items_attributes
  accepts_nested_attributes_for :items
end

Now when I instantiate a new invoice and a set of containing items, I want these items to know something about the invoice they belong to even before they are saved, so I can do something like this in my Item model:

class Item < ActiveRecord::Base
  belongs_to :invoice
  after_initialize :set_hourly_rate

  private 

    def set_hourly_rate
      if new_record?
        self.price ||= invoice.project.hourly_rate
      end
    end   

end

Right now, my code fails because the child (item) doesn’t know anything about its parent (invoice) during instantiation. Only after saving the invoice (and thus its nested items), it all works out. But I want to set a default value on each new item even before it gets saved.

How can this be done?

Thanks for any help.

Answer 1

You can add a callback on the invoice association, as follows:

class Invoice < ActiveRecord::Base
  # Code
  belongs_to :project
  has_many :items, :after_add => :set_item_price

  private

  def set_item_price(item)
    item.price = project.hourly_rate
  end  

end